Electron-beam lithography apparatus draws patterns onto a sample such as a mask, a wafer, or the like by exposure of an electron beam. Since an electron beam used for the drawing is susceptible to ambient circumstances including temperature, barometric pressure, and magnetism, the position of a beam spot, a focal point, a state of deflection, and a shaped beam size change over time. This causes drawing disorder or degradation of a precision in drawing. In order to correct the degradation of the drawing precision and draw patterns accurately, the electron-beam lithography apparatus must perform calibration from time to time.
In the electron-beam lithography apparatus, when calibrating the precision in drawing, a reference mark inscribed on a plane flush with a drawing plane is moved to a position immediately below the electron beam. The electron beam is irradiated to reference mark, and the calibration is carried out by optimizing the electron beam according to the intensity of electrons reflected from the reference mark or the intensity of electrons transmitted through the reference mark.
The frequency of calibration may be determined diversely, that is, may be determined as “monthly,” “weekly,” “daily,” “in units of a lot,” or “at every time of drawing.” An operator determines the calibration frequency and a calibration menu according to an ambient condition or the stability of the apparatus. A Change in temperature or magnetism is coped with by preparing a thermostatic chamber using a magnetic shield so as to stabilize temperature and humidity and prevent magnetic noise from entering. As for the change in barometric pressure, no measures have been taken in the apparatus in the past. In many cases, the change in barometric pressure has caused deformation of a vacuum tank and has been regarded as a factor of degradation of the precision of the apparatus.
Referring to FIG. 3, a conventional example will be outlined below. As the example, an electron-beam lithography apparatus of a variable-beam shape/stage lock-on control type is adopted. Noted is that the present invention is not limited to any specific type of electron-beam lithography apparatus.
An electron beam 11 radiated from an electron source 1 is irradiated to a first mask 4 after being aligned by a beam aligner 2. A beam aligner control circuit 3 controls the beam aligner 2. The electron beam transmitted via the first mask 4 passes through a shaping lens 7 and reaches a second mask 8. When the electron beam is irradiated to the second mask 8, a control computer 15 instructs a shaping deflection control circuit 6 to determine a dimension of the electron beam-shape. The shaping deflection control circuit 6 applies a voltage on the determined dimension to the shaping deflector 5, whereby a rectangular electron beam passes through the second mask 8 so that it will have the determined dimension on a wafer 13.
The electron beam 11 transmitted through the second mask is demagnified by a demagnification lens 9. A position of deflection determined by the control computer 15 is set in the deflection control circuit 16. A deflection signal according to the determined deflection position is applied from the control circuit 16 to a positioning deflector 10. Thereby the electron beam 11 is deflected to the determined position, and then irradiated onto the wafer 13 on a stage 14 through an objective lens 12.
The position of deflection to be determined by the control computer 15 is calculated based on the result of measurement of a current stage position and coordinates representing a target position of drawing. The current stage position is always measured using a laser length meter 17. The laser length meter 17 has a mirror disposed on the wall of a vacuum tank. When the vacuum tank deforms due to the influence of changes in barometric pressure, the result of measurement of the current stage position contains an error.
Patent Document 1 (Japanese Patent Laid-Open No. 2003-17394) describes that a vacuum tank included in a apparatus has a double structure in efforts to cope with the above error. Namely, the barometric pressure in an outer tank is held constant and the laser length meter is put in an inner tank in order to improve a precision. However, as far as the electron-beam lithography apparatus is concerned, since the electron-beam lithography apparatus includes a mechanism for transporting a mask substrate or a wafer to the inside of the apparatus and a mechanism for driving a stage or any other internal moving member, a vacuum tank having a completely double structure cannot be adopted. The adverse effect of deformation of the wall of the vacuum tank cannot be eliminated.
Moreover, according to Patent Document 2 (Japanese Patent Laid-Open No. 2003-124096), a barometric pressure is monitored and the irradiated position on a sample of the electron beam is corrected proportionally according to the monitored change in barometric pressure in order to improve the precision in drawing.
However, experiments conducted by the present inventor et al. have revealed that a change in the state of an electron beam does not have a linearly proportional relationship to a change in barometric pressure. FIG. 4 graphically shows changes in barometric pressure and variations in the irradiated position of an electron-beam spot in the conventional electron-beam lithography apparatus. The scale of a graph is manipulated so that the indication of the variations in the irradiated position of the electron-beam spot and the indicated of the changes in barometric pressure overlap one another in the right-hand part of the graph. In the left-hand part of the graph, the barometric pressure changes sharply, and the variations in the irradiated position of the electron-beam spot differ from the changes in barometric pressure.
This signifies that the variations in the position of the electron-beam spot don't have a linearly proportional relationship to the changes in barometric pressure. If an attempt is made to artificially bring about a large change in barometric pressure, which is so large as to break the linearly proportional relationship, in order to determine a correction formula, a large-scale experiment must be conducted. The idea is therefore unfeasible. On the other hand, when a computer is used to perform simulation so as to determine the correction formula, quite a high calculating precision is required. It is very hard to calculate a degree of deflection of a correcting precision, which is on the order of nanometers and required as a precision in correction, relative to a structure of several meters high. There is a high possibility that the required correcting precision cannot be ensured.